Tuesday, February 10, 2009

To AGC, or not to AGC

Way back at my last mapping appointment, I had my backup processor loaded with some test programs so I could have a listen to different settings that are available. I finally got around to trying the first one out. The only difference between it and my typical one is that AGC (Automatic Gain Control) is disabled in the test program. AGC is by default active when programming the Harmony processor, so any of you Harmony users most likely use AGC all the time.

AGC is defined by Advanced Bionics as "A dual loop gain and compression system. One loop adjusts the gain to soft level input while the second loop attenuates transient loud input. The dual AGC automatically adjusts patient sensitivity for optimal audibility and comfort." And "AGC automatically optimizes gain and compression to the listening environment. Disabling AGC results in peak clipping and may be situationally preferred by some listeners."

I spent a few hours switching back and forth between my normal program and the test.

Pros (AGC OFF):
  • No pumping. Pumping is a side effect of loud transients (e.g. dishes clattering) causing other concurrent continuous sounds to vary in volume. It is really only a minor annoyance when AGC is ON.
  • Transient peaks don't squash other sounds. When AGC is ON, loud bursts reduce overall volume quickly, with a several hundred millisecond release time delay. Other concurrent sounds, such as the voice of someone one is listening to, are momentarily lowered in volume which can affect understanding.
  • More natural sound in a setting without wide-ranging levels.
Cons (AGC OFF):
  • Transient peaks distort easily. The importance of this affect depends on the source. The distortion of a rapid transient peak, like a hand clap, is easily ignored. But with a loud talker at close range, the distortion reduced intelligibility.
  • Music quickly turns to a distorted messy mush of sound as volume increases. Awful.
  • Soft sounds are not made louder. Quiet speech and other desirable soft sounds are harder to hear well.

I don't think I will be using an AGC OFF setting very often. AGC handles a wide range of situations very well, with only a couple of shortcomings. I think it might be most useful in a controlled sound environment in which there are occasional transient bursts causing pumping when AGC is ON -- such as watching TV while one's child is banging on toys :)

I am almost certain that AGC as implemented in the AB Harmony processor is a full-band process. It compresses the whole signal. I think a future improvement would be to make it a multi-band process, which is what is used in most hearing aids. The benefit is much less pumping and squashing since only narrow bands are normally affected.

I will see if I can find any situations where disabling AGC results in enough improvement to garner a permanent slot in my processor.

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